The present invention relates to a die head assembly for a meltblown apparatus, and more particularly to a process and breaker plate assembly for producing bicomponent fibers in a meltblown apparatus.
A meltblown process is used primarily to form fine thermoplastic fibers by spinning a molten polymer and contacting it in its molten state with a fluid, usually air, directed so as to form and attenuate filaments or fibers. After cooling, the fibers are collected and bonded to form an integrated web. Such webs have particular utility as filter materials, absorbent materials, moisture barriers, insulators, etc.
Conventional meltblown processes are well known in the art. Such processes use an extruder to force a hot thermoplastic melt through a row of fine orifices in a die tip head and into high velocity dual streams of attenuating gas, usually air, arranged on each side of the extrusion orifice. A conventional die head is disclosed in U.S. Pat. No. 3,825,380. The attenuating air is usually heated, as described in various U.S. Patents, including U.S. Pat. Nos. 3,676,242; 3,755,527; 3,825,379; and 3,825,380. Cool air attenuating processes are also know form U.S. Pat. No. 4,526,733; WO 99/32692 and U.S. Pat. No. 6,001,303.
As the hot melt exits the orifices, it encounters the attenuating gas and is drawn into discrete fibers which are then deposited on a moving collector surface, usually a foraminous belt, to form a web of thermoplastic material. For efficient high speed production, it is important that the polymer viscosity be maintained low enough to flow and prevent clogging of the die tip. In accordance with conventional practice, the die head is provided with heaters adjacent the die tip to maintain the temperature of the polymer as it is introduced into the orifices of the die tip through feed channels. It is also known, for example from EP 0 553 419 B1, to use heated attenuating air to maintain the temperature of the hot melt during the extrusion process of the polymer through the die tip orifices.
Bicomponent meltblown spinning processes involve introducing two different polymers from respective extruders into holes or chambers for combining the polymers prior to forcing the polymers through the die tip orifices. The resulting fiber structure retains the polymers in distinct segments across the cross-section of the fiber that run longitudinally through the fiber. The polymers are generally xe2x80x9cincompatiblexe2x80x9d in that they do not form a miscible blend when combined. Examples of particularly desirable pairs of incompatible polymers useful for producing bicomponent or xe2x80x9cconjugatexe2x80x9d fibers is provided in U.S. Pat. No. 5,935,883. These bicomponent fibers may be subsequently xe2x80x9csplitxe2x80x9d along the polymer segment lines to form microfine fibers. A process for producing microfine split fiber webs in a meltblown apparatus is described in U.S. Pat. No. 5,935,883.
A particular concern with producing bicomponent fibers is the difficulty in separately maintaining the polymer viscosities. It has generally been regarded that the viscosities of the polymers passing through the die head should be about the same, and are achieved by controlling the temperature and retention time in the die head and extruder, the composition of the polymers, etc. It has generally been felt that only when the polymers flow through the die head and reach the orifices in a state such that their respective viscosities are about equal, can they form a conjugate mass that can be extruded through the orifices without any significant turbulence or break at the conjugate portions. When a viscosity difference occurs between the respective polymers due to a difference in molecular weights and even a difference in extrusion temperatures, mixing in the flow of the polymers inside the die head occurs making it difficult to form a uniform conjugate mass inside the die tip prior to extruding the polymers from the orifices. U.S. Pat. No. 5,511,960 describes a meltblown spinning device for producing conjugate fibers even with a viscosity difference between the polymers. The device utilizes a combination of a feeding plate, distributing plate, and a separating plate within the die tip.
There remains in the art a need to achieve further economies in meltblown processes and apparatuses for producing bicomponent fibers from polymers having distinctly different viscosities.
Objects and advantages of the invention will be set forth in the following description, or may be apparent from the description, or may be learned through practice of the invention.
The present invention relates to an improved die head assembly for producing bicomponent fibers in a meltblown spinning apparatus. It should be appreciated that the present die head assembly is not limited to application in any particular type of meltblown device, or to use of any particular combination of polymers. It should also be appreciated that the term xe2x80x9cmeltblownxe2x80x9d as used herein includes a process that is also referred to in the art as xe2x80x9cmeltspray.xe2x80x9d
The die head assembly according to the invention includes a die tip that is detachably mounted to an elongated support member. The support member may be part of the die body itself, or may be a separate plate or component that is attached to the die body. Regardless of its configuration, the support member has, at least, a first polymer supply passage and a separate second polymer supply passage defined therethrough. These passages may include, for example, grooves defined along a bottom surface of the support member. The grooves may be supplied by separate polymer feed channels.
The die tip has a row of channels defined therethrough that terminate at exit orifices or nozzles along the bottom edge of the die tip. These channels receive and combine the first and second polymers conveyed from the support member.
An elongated recess is defined in the top surface of the die tip. This recess defines an upper chamber for each of the die tip channels. An elongated upstream breaker plate and an elongated downstream breaker plate are removably supported in a stacked configuration within the recess. Each of the breaker plates has pairs of adjacent holes defined therethrough. The holes in the stacked breaker plates are aligned such that a pair of the aligned holes is disposed in each upper chamber of the die tip channels. In one embodiment, the upstream breaker plate has a top surface that lies flush with, or in the same plane as, the upper surface of the die tip. In this embodiment, the top surface of the die tip is mountable directly against the underside of the support member. The holes in the upstream breaker plate are spaced apart and sized so that they align with the separate supply passages or grooves defined in the underside of the supply member. In this manner, the polymers are prevented from crossing over or mixing between the holes, and are maintained completely separate as they are conveyed into the breaker plates.
A filter device, such as a mesh screen, is disposed in the recess, for example between the upstream and downstream breaker plates. The filter device serves to separately filter the polymers conveyed through the breaker plate holes prior to the polymers entering and combining in the die tip channels.
At each of the channels, the first and second polymers are conveyed from the support member supply grooves or passages and flow through respective separate holes in the upstream breaker plate. The polymers flow through and are separately filtered by the filter device. The polymers finally flow through the aligned holes in the downstream breaker plate and into the die tip channels. In the channels, the polymers merge into a single molten mass having an interface or segment line between the separate polymers prior to being extruded as bicomponent polymer fibers from the die tip orifices.
The breaker plate holes may take on various configurations and sizes. In one embodiment, each hole of the pair of holes in the upstream breaker plate have the same diameter. The holes in the downstream breaker plate may also have the same diameter, and this diameter may be the same as that of the holes of the upstream breaker plate. In an alternative embodiment, the individual holes of the pair of holes in the upstream breaker plate may have different diameters. The downstream breaker plate holes may have correspondingly sized different diameters. It should be readily apparent that various combinations of hole sizes or patterns may be configured in the breaker plates.
The invention will be described in greater detail below with reference to the appended figures.